The present invention relates to an amplifier for an analog-to-digital (AD) converter that amplifies a useful signal prior to digitization by the AD converter.
One of the disadvantages of a system with an AD converter is that when the resolution is too low, the quantization of the input signal may be noticeable and cause interference. A resolution of 8-bits (256 different discrete values) has an accuracy sufficient for many applications in image processing. However, this quantization can often be seen in the image. For example, if the analog values correspond to the brightness values of a line-scanned image in which the brightness changes continuously in the horizontal direction, then only 256 different brightness values can be reconstructed from the 8-bit digitized signal for this example. The transitions from one brightness value to the next occur at the same position in every line, which appears to the human eye as a vertical stripe or an abrupt change in brightness.
One approach to solving this problem provides for superimposing a noise signal on the useful signal prior to quantization. The majority of the noise power is above the frequency range of the useful signal. Although this technique reduces the signal-noise ratio of the system, it results in an improved digitally reconstructed image. A random signal is superimposed on each analog value to be digitized, and the level of this signal is low in comparison with the level of the useful signal, ensuring that the same analog signals, depending on the amplitude of the random signal can be imaged at different discrete values at the output of the AD converter. The brightness values that come from one column of the scanned image are therefore not necessarily imaged on the same discrete values. In particular, analog values located in the boundary area between two quantization stages are imaged in random fashion at the higher or lower quantization levels. In the image obtained from the digitized values, the change from one brightness level to the next in different lines can occur at different positions that differ by a few image points so that the brightness change is no longer perceived as a jump by the human eye.
The noise signal whose average power is supposed to be constant is produced in known amplifiers by a resistance network or an additional adder amplifier, connected downstream of the output of the amplifier and upstream of the input of the AD converter. The resistance network has a particular disadvantage that it increases the output resistance of the amplifier while the AD converter requires a low output resistance of the amplifier located upstream, since otherwise the input signal errors of the AD converter increase at high frequencies. A disadvantage of the additional adder-amplifier is that the current draw and area occupied by the system increase.
In known amplifiers of this kind, the noise signal is superimposed on the input signal prior to amplification by the amplifier and as a result is amplified as well. Since the amplification of the amplifier is intended to be variable and the average power of the noise signal supplied to the AD converter is intended to remain constant, this solution requires that the noise signal always be adapted to the respective amplification.
Briefly, according to the present invention, an amplifier has a first amplifier stage with a first input terminal to which an input signal is applied and with an output terminal that is connected to an input terminal of a second amplifier stage located downstream. The second amplifier stage provides an output signal that is fed back to a second input terminal of the first amplifier stage through a feedback path, which preferably contains a passive network. The amplification of this combination of the first and second amplifier stages with feedback depends essentially on the parameters of the network in the feedback path. The amplifier according to the invention also has a noise source that is connected in parallel with the first amplifier stage, and is also connected to the input terminal of the second amplifier stage. The level of a signal component at the output of the second amplifier stage depends on the parameters of the network in the feedback branch and the amplification of the first amplifier stage. These values are variable and can be adjusted such that the level of the signal component provided by the noise source is independent of the amplification of the amplifier.
One embodiment the passive network comprises a voltage divider, with one tap of the voltage divider connected to the second input terminal of the first amplifier stage. A k-fold multiple of the output signal at the second amplifier stage is applied to the second input terminal of the first amplifier stage, with the feedback factor k depending on the voltage divider ratio.
The amplification of the first amplifier stage may also be adjustable as a function of the feedback factor k. In one embodiment, the first amplifier stage contains a plurality of amplifiers connected in parallel that can be switched as a function of the feedback factor k. The amplifiers of the first amplifier stage are therefore preferably designed as differential amplifiers.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.